What Is the Nyquist Limit?
The Nyquist limit is defined as half of the sampling frequency. The Nyquist limit sets the highest frequency that the system can sample without frequency aliasing. In a sampled data system when the input signal of interest is sampled at a rate slower than the Nyquist limit (fIN > 0.5fSAMPLE), the signal is effectively “folded back” into the Nyquist band. The signal thus appears to be at a lower frequency than it actually is. This unwanted signal is indistinguishable from other signals in the desired frequency band (fSAMPLE/2). For example, if your system samples the ADC at 2.048kHz, the Nyquist limit for that system is 1.024kHz. If an input signal is applied at 2.56kHz, the input signal is indistinguishable from a 0.512kHz signal (2.56kHz – 2.048kHz) due to frequency aliasing. No amount of clever digital signal processing can restore the information lost from frequency aliasing. The only way to know what you have sampled is to physically restrict the input bandwidth prior to sampling.
